Multi output dc/dc converter for liquid crystal display device

ABSTRACT

A liquid crystal display (LCD) system comprising means for generating a number of LCD drive voltages with values symmetrical with respect to a predetermined voltage value, said means having a configuration of buffer capacitors to provide each of the LCD drive voltages with a buffer capacitance, the LCD system further comprising an LCD driver circuit with matrix switching and control means to supply the terminals of an LCD panel with voltages corresponding to said LCD drive voltages, resulting in a proper light level of the pixels of the LCD panel. To define the LCD drive voltage values, at least one charge pump unit is provided with at least one pump capacitor and switching elements, which at least one charge pump unit is connected to the buffer capacitors.

The invention relates to a liquid crystal display (LCD) system,comprising means for generating a number of LCD drive voltages withvalues symmetrical with respect to a predetermined voltage value, saidmeans having a configuration of buffer capacitors to provide each of theLCD drive voltages with a buffer capacitance, the LCD system furthercomprising an LCD driver circuit with matrix switching and control meansto supply the terminals of an LCD panel with voltages corresponding tosaid LCD drive voltages, resulting in a proper light level of the pixelsof the LCD panel.

In practice LCD modules are required which are fed only by a givenvoltage source, particularly a battery, or with a voltage derived from abattery and have a given format for the pictures on the panel. One ofthe most important applications for small LCD systems is in cellularphones; the voltage supply source in such applications is a battery.Mostly this battery is a single Li-ion cell or is formed by Ni-typecells, such as nickel-cadmium (NiCd) or nickel-metal hydride (Ni) cells.In practice, the battery voltage ranges from 4.2 to 2.5 V with Li-typebatteries and from 4.8 to 0.9 V with Ni-type batteries when fullycharged and gradually becoming fully discharged. The required LCD drivevoltages is to be generated from this single battery supply voltage. Thestandby power consumption is, besides picture quality, one of the mostimportant parameters for cellular phones. The display is on all thetime, and thus power supply of the display is a matter of concern.Therefore, the conversion of a single battery voltage into a number ofwell-controlled LCD drive voltages needs to be done with relatively highefficiency in order to keep the standby power consumption low.

An LCD system as described in the opening paragraph is known from U.S.Pat. No. 5,986,649. A charge pump technique is applied in the means forgenerating a number of symmetrical LCD voltages in said document toobtain well defined voltages V3 and −V3, whereas well-definedintermediate voltages V2, VC and −V2 are generated by means of driverelements including resistors R1-R4, operational amplifiers OP1 and OP2,and a serial configuration of capacitors C1-C4. Although this knownsystem generates well-defined LCD drive voltage, the application of suchdriver elements in combination with load currents occurring in theseamplifiers results in a dissipation of energy, particularly in theoperational amplifiers, which will not always be acceptable in practice.

The purpose of the invention is to provide an LCD system wherein thedissipation in the means for generating the LCD drive voltages isstrongly reduced in comparison with the known configuration.

Therefore, according to the invention, the LCD system as described inthe opening paragraph is characterized in that at least one charge pumpunit with at least one pump capacitor and switching elements isconnected to the buffer capacitors.

The combination of buffer capacitors together with the application ofcharge pump technology at the output of the buffer capacitors rendersthe exchange of charge between the several buffer capacitors with highefficiency possible. The use of buffer amplifiers, as in the case of theabove prior art, is superfluous now, so that less power will bedissipated in the LCD system.

The buffer capacitor configuration can be realized in different ways.The above prior art document teaches a serial configuration of buffercapacitors arranged between the output terminals of a single supplyvoltage device with a buffer capacitor between each of the LCD drivevoltages. A further possible buffer capacitor configuration is a starconfiguration, where the buffer capacitors are arranged between therespective LCD drive voltages and a common point, for example ground orthe LCD drive voltage with respect to which the other LCD drive voltageshave symmetrical values. Combinations of a serial configuration and astar configuration of buffer capacitors are also possible.

In a more particular embodiment, the LCD system is characterized in thatthe means for generating a number of LCD drive voltages comprises aDC/DC converter to supply an output voltage for the configuration ofbuffer capacitors, and that a charge pump unit is provided comprising atleast one first pump capacitor and respective switches to define a firstgroup of LCD drive voltage differences and at least one second pumpcapacitor and respective switches to define, in combination with the atleast one first pump capacitor and respective switches, a second groupof LCD drive voltage differences, the latter voltage differences beingsubstantially equal to the LCD drive voltage differences of the firstgroup. In another particular embodiment, the LCD system is characterizedin that the means for generating a number of LCD drive voltagescomprises a DC/DC converter to supply an output voltage for theconfiguration of buffer capacitors, and that a first charge pump unit isprovided comprising at least one pump capacitor and respective switchesto define a first group of LCD drive voltage differences, and a secondcharge pump unit comprising at least one pump capacitor and respectiveswitches to define a second group of LCD drive voltage differences.Combinations of the two embodiments are possible.

An LCD system will be provided particularly for cellular phones, inwhich the means for generating a number of LCD drive voltages comprisesa DC/DC up-converter fed by a battery voltage to generate the LCD drivevoltages. Nevertheless, a DC/DC down-converter fed by a battery voltageto generate the LCD drive voltages may alternatively be applied. Thismay have advantages because down-conversion provides less output ripplethan up-conversion. The applicable lower capacitance values can lead tosmaller dimensions and a lower cost price. Of course, the choice ofup-conversion or down-conversion will have consequences for therealization of control circuits of the charge pump unit.

The invention will be apparent from and elucidated with reference to theexamples as described in the following and to the accompanying drawing.In this drawing

FIG. 1 is a basic diagram of an LCD system;

FIG. 2 shows an LCD system with driver elements according to the stateof the art;

FIG. 3 shows part of an LCD system with a possible generation of themidpoint voltage VC;

FIG. 4 shows a non-applicable extension of the system in FIG. 3;

FIG. 5 shows a first embodiment of an LCD supply voltage generator witha DC/DC up-converter, in which generator charge pump technology isapplied for voltage generation and reduction of energy consumptionaccording to the invention;

FIG. 6 shows a second embodiment of such a voltage generator with analternative implementation of the charge pump unit;

FIG. 7 shows a third embodiment of such a voltage generator with asecond charge pump unit for providing additional drive voltages for theLCD system; and

FIG. 8 shows a fourth embodiment of an LCD supply voltage generator witha DC/DC down-converter and an implementation of the charge pump unit asillustrated in FIG. 7.

FIG. 1 is a basic diagram of an LCD system with means for generating anumber of symmetrical LCD voltages in the form of an ICD supply voltagegenerator 1 fed by a battery 2 and LCD driver circuit 3 to supply theterminals of an LCD panel 4 with the LCD drive voltages. The LCD drivercircuit 3 comprises matrix switching and control means in a knownmanner. A matrix of 68 rows and 98, or for a color panel 3×98, columnsis a practical configuration for a cellular phone. The LCD systemfurther comprises a processor with a control algorithm to control theabove hardware; this processor is not indicated in the Figures.

As an example, the matrix switching and control means could require thefollowing LCD drive voltages: V3=15.8 V; V2=10.7 V; V1=9.3 V; VC=7.9 V;MV1=6.5 V; MV2=5.1 V and MV3=0 V. These values are indicated in FIG. 1.4 stacked voltages of 1.4 V centered around VC (Vcommon) that are inturn extended at both sides with 5.1 V can be recognized from thesevalues. For the LCD, the voltage level to ground is of no relevance; anylevel other than MV3 could be chosen as zero reference. The requiredvoltage range exceeds that of the voltage provided by the battery 2,which supplies, for example, fully charged, a voltage of max. 4.8 V, sothat some form of voltage up-conversion must be applied in the LCDsupply voltage generator 1. The LCD drive voltages for the LCD drivercircuit 3 need to be well-controlled and independent of the batterycharge status.

Although the load formed by the LCD panel 4 is capacitive, this does notmean that the LCD drive voltages delivered to the driver circuit 3 donot have to provide a DC current. However, the DC component of the drivevoltages delivered by the LCD driver circuit 3 must be zero. This isachieved by alternately driving the LCD driver circuit 3 with the samevoltage but with opposite polarity. A practical way of doing so impliesthe existence of complementary drive voltages. The above drive voltages,which have values symmetrical with respect to the value of VC, canrealize this. For example, the voltage differences V1−VC and VC−MV1provide an equal current flow into and from the terminal VC, as will beshown in the further description.

The LCD supply voltage generator 1 has to deliver the drive currents.Although the load is capacitive, the net currents to be delivered by thesupply voltage generator are not zero. The most significant currents arethose from V1 via a respective load to VC and from VC via a suchlikeload to MV1. In a practical LCD system, large unipolar current pulses ofthe order of magnitude of 100 mA will flow from V1 to VC andsubsequently from VC to MV1. These current pulses may sum up to anaverage current flowing from one supply terminal into an other of, forexample, 250 μA.

FIG. 2 shows an example of an LCD system wherein the LCD drive circuit 3and the LCD panel 4 are replaced by an equivalent diagram 5,illustrating the average load currents by means of arrows. Short peakcapacitive load currents are subsequently generated in an adequatelychosen sequence in the LCD drive circuit 3. This means that the loadcurrents are flowing in different time slots depending on the driverscheme in the LCD drive circuit 3. This sequence is realized by means ofthe control algorithm of the processor in the LCD system.

As an example, the average load currents may be: V3→V1=12.5 μA;V3→MV1=12.5 μA; V2→VC=0.50 μA; and V1→VC=250 μA. The symmetrical otherones are the same.

In the example of FIG. 2, the output drivers 6-10 in the LCD supplyvoltage generator 1 provide the LCD drive voltages V2, V1, VC, MV1, andMV2. For practical reasons these output drivers are fed with the highestand lowest voltages V3 and MV3. However, more adequate supply voltagesmay be chosen.

As was stated above, the average current is composed of a large numberof short peaks flowing in different time slots that depend on the driverscheme. The existence of the large current pulses is caused by theapplication of voltage steps across the capacitive loads. Theapplication of decoupling or buffer capacitors 11-16 at the output ofthe driver 6-10 relaxes the required performance of these drivers,because the large current peaks are provided by the capacitors in thiscase, and it is only the drivers 6-10 that must supply the averagecurrent. In this case, the drivers may have a low current drivecapability and a higher output impedance, which means smaller circuitsin an IC.

In the system of FIG. 2, the average load current is supplied via theoutput drivers 6-10, which drivers provide the LCD drive voltages V2,V1, VC, MV1, and MV2. Power is dissipated in each of the drivers 6-10 independence on its supply voltage, in this case the values V3 and MV3,and the load currents. Even with a more complex implementation, wherethe smallest possible supply voltage for each driver is used, the powerdissipation remains a point of concern.

In LCD systems, the ac operation conditions imply load currents that aresubstantially equal for sets of two load current supply sources. So, theload currents from V1 to VC and subsequently from VC to MV1 effectivelyyield a net current of zero in the VC terminal. When considering theload current of VC, the use of decoupling capacitors implies that the DCimpedance of the VC drive voltage may be rather high since the averagecurrent is zero. This makes it possible to apply two resistors 17 and 18for the generation of VC instead of output drivers. Such a generation ofthe midpoint voltage VC is shown in FIG. 3. A voltage converter 19generates the voltages VI and MV1. Although the application of simpleresistors instead of drivers is a cheap solution and diminishes thedissipation of energy by the omission of drivers, this solution is notvery efficient because the generation of the other LCD drive voltagesmeets with further difficulties, as will be explained with reference toFIG. 4.

As is shown in FIG. 2, the voltages V2, V1, VC, MV1, and MV2 can begenerated with DC drivers 6-9 aided by decoupling capacitors 11-16 forproviding the instantaneous very high load peaks. When no DC currentneeds to be delivered, high-ohmic resistors may already provide theproper DC voltage. This is the case for VC as illustrated in FIG. 3.With four equal voltages V2-V1, V1-VC, VC-MV1, and MV1-MV2 as required,this measurement can only be made if the DC load current in theterminals for V1, VC, and MV1 is zero. This, however, is not the case.When looking at FIG. 2, the load currents from V1 to VC and subsequentlyfrom VC to MV1 are not supplied other than via the respective drivers.As illustrated in the above example for the load currents, the currentdelivered from V2 to VC and subsequently from VC to MV2 does not cause asubstantial net current flow into VC. In FIG. 4, an LCD voltagegenerator is depicted in which this no-current load condition of fourequal LCD voltage differences can be answered with high-ohmic resistors17-20. However, the actual current load would change the DC potential ofthe several drive voltages. The application of low-ohmic resistors isnot acceptable because of energy losses and the application of resistorswith different values for providing the appropriate voltages is onlypossible with well-defined and constant currents. This is not possiblesince the load current of an LCD panel is determined by the picturecontent. Departing from four equal voltages of 1.4 V at no-current load,the two middle capacitors 13 and 14 would be discharged and the twoneighboring capacitors 12 and 15 would be charged due to the loadcurrent, so that the voltages V1-VC and VC-MV1 would be lower than 1.4 Vand the voltages V2-V1 and MV1-MV2 would be higher than 1.4 V. It is tobe noted that the voltage up-converter 21 generates the voltages V2 andMV2.

As can be recognized from FIG. 4, with equal capacitor values, the LCDsupply voltage generator delivers half the load current via thecapacitors 12 and 15. The inner capacitors 13 and 14 are discharged andthe neighboring capacitors 12 and 15 are charged. This means that abetter approach would be the application of driver circuits for thedefinition of the several de voltages. However, that is still not anenergy-efficient solution.

According to the invention, the application of charge-pump technique canprovide a redistribution of charge, i.e. charge can be transferred fromthe two charged capacitors 12 and 15 to the two discharged capacitors 13and 14. An LCD system requiring a charge pump unit 22 in the form of acombination of a single charge pump capacitor 23 and switches 24-27 isdepicted in FIG. 5. The pump capacitor 23 is subsequently connected viasaid switches 2427 in parallel to the stacked capacitors 12-15 andtransfers charge from one capacitor to the other. The moment a drivevoltage should be disturbed because of a certain load current, the pumpcapacitor will restore the respective drive voltage. The resistancevalue may be high in this system. As was found in practice, up to nowonly the pump technique has provided the correct voltage distributionunder load conditions such that the resistors can even be omitted.Energy is transferred from one capacitor to the other, and the currentto be supplied from the DC/DC converter can theoretically be half theoriginal one.

It is to be noted that, as is the case in the embodiment of FIG. 4, thevoltage up-converter 28 generates the voltages V2 and MV2. The voltagesV1, VC, and MV1 are obtained by a pump technique instead of resistors,as in the embodiment of FIG. 4.

In practice, it may be advantageous to apply more pump capacitors forreasons of ripple, available component values, preferred switchingfrequency, etc. A configuration using two pump capacitors 29 and 30 isdepicted in FIG. 6. This configuration shows a first group with pumpcapacitor 29 and switches 24 and 25 and a second group with pumpcapacitor 30 and switches 26 and 27.

In FIG. 6, no adequate measures are taken to define the midpoint dcvoltage (i.e. VC). Again, this can be achieved by the application of adriver circuit or a pair of resistors.

In this specific situation of the load, only some possible asymmetrycaused by leakage, circuit load, etc., must be accommodated. For largerasymmetry it is better to create an overlap of the two switch-capacitorgroups. This somewhat resembles twice the situation as depicted in FIG.5 or, for example, a situation in-between where only the two middlecapacitors 13 and 14 are connected via the additional switches to thepump capacitors 29 and 30 of the two groups. This implies an additionalcharge transfer from one pump capacitor to the other as indicated by thedashed arrows in FIG. 6.

Up to now, no attention has been paid to the outer voltages of 5.1 V.Again, these voltages can be derived by charge pump technology from anavailable voltage in the system. Such an adequate voltage is availablebetween nodes V2 and MV2. Therefore, the embodiment in FIG. 5 isextended by the addition of an extra pump capacitor 31 and switches32-34 as depicted in FIG. 7.

FIG. 8 shows substantially the same embodiment as FIG. 7. However,instead of an up-converter to derive the drive voltages V2 and MV2, adown-converter 35 is applied to derive the drive voltages V1 and MV1.This embodiment may have advantages as down-conversion can be realizedmore cheaply than up-conversion. The drive voltage VC is defined bymeans of the pump capacitor 29 and the switches 25 and 26, while thedrive voltages V3, V2, MV2, and MV3 are defined by both pump capacitors29 and 31 and switches 24, 27 and 32-34.

It will be clear that the sequence of load currents and the controlthereof as well as the control of the switches of the charge pump unitcan be realized by means of a processor which forms part of the LCDsystem. The sequence of the load currents can be coupled to the controlof the switches of the charge pump unit. Furthermore, the control of theLCD system may be synchronous or asynchronous, at the same frequency orat different frequencies. This may have advantages with respect topicture artefacts.

The invention is not restricted to the described embodiments;modifications within the scope of the following claims are possible.Particularly, the charge pump unit may be realized in different waysthrough the arrangement of more pump capacitors and other configurationsof switches. More charge pump units may be provided. Furthermore, forexample, the configuration of FIG. 6 may be combined with that of FIG.7, resulting in an LCD system with two charge pump units with a total ofthree pump capacitors, each operable with a set of switches: a firstpump capacitor 29 and switches 24 and 25 for defining LCD drive voltagesV2, V1, and VC, a second pump capacitor 30 with switches 26 and 27 fordefining LCD drive voltages VC, MV1, and MV2, and a third pump capacitor31 with switches 32, 33, and 34 for defining the LCD drive voltages V3and MV3. In general, the LCD system in this case is characterized inthat the means for generating a number of LCD drive voltages comprises aDC/DC converter to supply an output voltage for the configuration ofbuffer capacitors, and that a first charge pump unit is providedcomprising at least one first pump capacitor and respective switches todefine a first group of equal LCD drive voltage differences and at leastone second pump capacitor and respective switches to define, incombination with the at least one first pump capacitor and respectiveswitches, a second group of equal LCD drive voltages, the latter voltagedifferences being equal to the LCD drive voltage differences of thefirst group, and a second charge pump unit comprising at least one thirdpump capacitor and respective switches to define an additional group ofequal LCD drive voltage differences.

It is a constraint relating to liquid crystals that drive voltages mustbe applied that have an average value of zero. For this, a number ofdrive voltages that have substantially symmetrical values around VC needto be made available; the examples in the Figures and in the descriptionoffer an LCD system with 4 substantially equal LCD drive voltagedifferences around midpoint VC. It is to be understood that this systemmay be extended to systems that provide more than 4 of such voltagedifferences, particularly for color LCDs.

Although the examples in the Figures and description show a seriesconnection of buffer capacitors for keeping the LCD drive voltagessubstantially constant when the related terminals are subject to somecurrent, alternative buffer capacitor configurations as indicated in theintroductory part of the description are equally possible.

It may further be noted that the type of DC/DC converter is irrelevant.The converter may be inductive (up, down and up/down) or capacitive; inthe latter case charge pump techniques will be applied. The choice ofconverter will be determined by costs, actual input voltage range, andrequired efficiency.

1. Liquid crystal display (LCD) system, comprising means for generatinga number of LCD drive voltages with values symmetrical with respect to apredetermined voltage value, said means having a configuration of buffercapacitors to provide each of the LCD drive voltages with a buffercapacitance, the LCD system further comprising an LCD driver circuitwith matrix switching and control means to supply the terminals of anLCD panel with voltages corresponding to said LCD drive voltages,resulting in a proper light level of the pixels of the LCD panel,characterized in that at least one charge pump unit with at least onepump capacitor and switching elements is connected to the buffercapacitors.
 2. LCD system according to claim 1, characterized in thatthe means for generating a number of LCD drive voltages comprises aDC/DC converter to supply an output voltage for the configuration ofbuffer capacitors, and that a charge pump unit is provided comprising atleast one first pump capacitor and respective switches to define a firstgroup of LCD drive voltage differences and at least one second pumpcapacitor and respective switches to define, in combination with the atleast one first pump capacitor and respective switches, a second groupof LCD drive voltage differences, the latter voltage differences beingsubstantially equal to the LCD drive voltage differences of the firstgroup (FIG. 6).
 3. LCD system according to claim 1, characterized inthat the means for generating a number of LCD drive voltages comprises aDC/DC converter to supply an output voltage for the configuration ofbuffer capacitors, and that a first charge pump unit is providedcomprising at least one pump capacitor and respective switches to definea first group of LCD drive voltage differences, and a second charge pumpunit comprising at least one pump capacitor and respective switches todefine a second group of LCD drive voltage differences (FIGS. 7 and 8).4. LCD system according to claim 1, characterized in that the means forgenerating a number of LCD drive voltages comprises a DC/DC converter tosupply an output voltage for the configuration of buffer capacitors, andthat a first charge pump unit is provided comprising at least one firstpump capacitor and respective switches to define a first group ofsubstantially equal LCD drive voltage differences and at least onesecond pump capacitor and respective switches to define, in combinationwith the at least one first pump capacitor and respective switches, thesame group of substantially equal LCD drive voltages (FIG. 6).
 5. LCDsystem according to claim 1, characterized in that the means forgenerating a number of LCD drive voltages comprises a DC/DC converter tosupply an output voltage for the configuration of buffer capacitors, andthat a first charge pump unit is provided comprising at least one firstpump capacitor and respective switches to define a first group of LCDvoltage differences and at least one second pump capacitor andrespective switches to define, in combination with the at least onefirst pump capacitor and respective switches, a second group of LCDdrive voltages, the latter voltage differences being substantially equalto the drive voltage differences of the first group, and a second chargepump unit comprising at least one third pump capacitor and respectiveswitches to define an additional group of substantially equal LCD drivevoltage differences (combination of FIGS. 6 and 7).
 6. LCD systemaccording to claim 2, characterized in that the means for generating anumber of LCD drive voltages comprises a DC/DC up-converter fed with abattery voltage so as to generate the LCD drive voltages (FIGS. 5-7). 7.LCD system according to claim 2, characterized in that the means forgenerating a number of LCD drive voltages comprises a DC/DCdown-converter fed with a battery voltage so as to generate the LCDdrive voltages (FIG. 8).